Glioblastoma, the grade IV astrocytoma, is the most malignant and most common primary adult brain cancer (Furnari et al., 2007). The highly heterogenous and infiltrative nature of these tumors makes complete resection virtually not possible and hence despite the postoperative treatment modalities, the median survival is very poor. While postoperative radiotherapy alone provided very small survival advantage, the addition of nitrosourea-based chemotherapy to radiotherapy gave modest benefit of increase in 1-year survival rate of 6%, such as from 40% to 46% and a 2-month increase in median survival time (Stewart, 2002). A 5-year analysis of the EORTC-NCIC trial showed addition of concomitant and cyclical adjuvant temozolomide, a DNA alkylating agent, to standard postoperative radiotherapy improved median survival and 2-year survival significantly relative to postoperative radiotherapy alone (Stupp et al., 2009). The overall survival rate was 27.2% at 2 years, 16.0% at 3 years, 12.1% at 4 years and 9.8% at 5 years with temozolomide versus 10.9%, 4.4%, 3.0%, and 1.9% respectively with radiotherapy alone (Stupp et al., 2009). In addition, the median survival increased to 14.6 months from 12.1 months (Stupp et al., 2009). More importantly, methylation of MGMT promoter was found to be the strongest predictor for outcome and benefit from temozolomide chemotherapy (Stupp et al., 2009). Analysis of progression-free survival showed an advantage only for patients whose tumor had a methylated MGMT promoter who were treated with temozolomide and radiotherapy.
These findings suggest the existence of specific gene signatures which will identify sub classes of patients with better response to temozolomide chemotherapy.
In the present invention a prospective study is carried out where patients were selected with standard inclusion/exclusion criteria and subjected to uniform treatment protocol, which included maximal safe resection of the tumor followed by radiotherapy with concomitant and cyclical adjuvant temozolomide therapy. The patients' survival data was correlated with tumor gene expression profile for identifying genes whose expression signature could predict survival. We report here our observations regarding influence of high topoisomerase II alpha (TOP2A) transcripts in GBM on patient survival. We further demonstrate by functional studies that temozolomide inhibits TOP2A activity in vitro and silencing of TOP2A in glioma cells rendered them temozolomide resistant. An assessment of TOP2A and other isoforms of topoisomerase across the grades of astrocytoma were also performed to elucidate their role in glioma.